To provide desired amplitude and phase characteristics, antenna reflectors are often shaped, i.e., the surface shape of the antenna is changed from a conic section (parabola, paraboloid, ellipse, ellipsoid, hyperbola, or hyperboloid) to a new curvature or shape. This shaping feature is used advantageously in a commonly-owned patent entitled, "Compact Antenna Range Employing Shaped Reflectors", U.S. Pat. No. 4,688,325, issued on Aug. 25, 1987. The shaping of antenna reflectors is also discussed in an article entitled, "Minimum-Noise Maximum-Gain Telescopes And Realization Method For Shaped Asymmetric Surfaces", by Sebastian VonHoerner, appearing in the I.E.E.E. Transactions on Antennas and Propagation, Volume AP-26, No. 3, May 1978, pages 464 through 471.
In a typical antenna panel fabrication operation, a flexible metal panel or skin is securely clamped to a solid bonding fixture having a predetermined shape. A honeycomb backup structure is glued to the metal skin and then a second metal panel or skin is glued to the exposed side of the honeycomb structure. Thus a sandwich is formed with the honeycomb structure between two flexible sheets of metal. After having been clamped to the bonding fixture for a predetermined curing time, the metal sheets and the honeycomb fixture are permanently deformed into the shape defined by the bonding fixture. The fabricated panel is then released from the bonding fixture, attached to a rigid frame, and arranged with other antenna panels to form a complete antenna reflector surface.
In lieu of using the solid bonding fixture, antenna panels can be advantageously fabricated using an adjustable bonding fixture disclosed and claimed in the co-pending commonly owned patent application entitled, "Adjustable Bonding Fixture For Antenna Panel Fabrication", Ser. No. 885,515, filed July 14, 1986 now U.S. Pat. No. 4,731,144. Using the adjustable bonding fixture, antenna panels are fabricated by laying a blank plastic or metal sheet over vertical adjustment rods threaded into a base plate. The adjustment rods are positioned at a predetermined distance above the base plate to provide the desired antenna panel shape. A honeycomb structure is then glued over the metal sheet, and a second metal sheet is glued to the exposed side of the honeycomb structure. The base plate and the entire antenna assembly are then enclosed and a vacuum is drawn within the enclosure. The vacuum causes the two sheets and the honeycomb structure to be formed into a shape defined by the tops of the adjustment rods. After the adhesive has cured, the antenna panel is permanently deformed into the desired shape. The antenna panel is then attached to a rigid frame and arranged with other panels to form a complete antenna reflector surface.
In lieu of the metal-honeycomb-metal sandwich, an antenna panel can also be formed by clamping a single metal sheet to the bonding fixture, and gluing a kerfed channel frame to the metal skin. The kerfed frame is a channel member with slits cut into it to make it flexible. After the glue has cured, the assembly is released from the bonding fixture. The skin, which was deformed by the clamps into the proper shape, is now held in that shape by adhesion to the kerfed channel. The kerfed channel assembly operation can be performed using either the well-known solid bonding fixture or the adjustable bonding fixture disclosed in the commonly-owned patent application.
The primary disadvantage of both of these antenna panel fabrication techniques is that neither provides a means to improve panel surface accuracy after the panel is released from the bonding fixture. Both of these techniques rely primarily on the conformance of the panel skin to the shape (i.e., surface curvature) defined by the solid or adjustable bonding fixture. There is no provision to improve the panel surface accuracy once the panel is released from the mold. Further, there are no means to compensate for the expected and slight springback of the panel after its release from the mold. Thus using these available antenna panel fabrication techniques, it is difficult to guarantee perfect conformance of the skin to the mold surface, and it is impossible to eliminate or compensate for the spring-back that occurs after the panel is released from the mold.
A complete antenna dish consists of several panels, fabricated as discussed above, assembled together in a predetermined pattern. The panels are shaped such that when assembled together they form the desired dish shape. The total surface accuracy of the antenna reflector depends substantially on the surface accuracy of each individual panel and the assembled accuracy of these panels. Even if the antenna panels are fabricated and assembled to a desired accuracy, during its lifetime the antenna is subjected to environmental effects such as wind, ice, and snow that have a tendency to distort the shape of the dish and degrade dish performance. Further, as the operating frequencies of the antenna dish increase, the surface accuracy of the dish plays a more important role in the performance of the antenna. At the extra-high frequency (EHF) range (18 to 90 GHz) the surface accuracy of the antenna dish must be maintained below 0.005 inches root means square (RMS) for acceptable performance.
It is well-known in the prior art to provide an adjustment device at each corner of each antenna panel, between the panel and the rigid frame to which it is mounted. When the panels are assembled to form a complete reflector, the distance between the frame and the panel is adjusted to ensure that each panel is flush with the adjacent panel.